1. Field of the Invention
The present invention is directed to a process for producing an aluminum nitride sintered product with a high thermal conductivity at a relatively low sintering temperature.
2. Description of the Prior Art
Aluminum nitride (AlN) is an excellent material having a high thermal conductivity, insulation resistance and a low thermal expansion coefficient, etc. However, since aluminum nitride is a covalent bonding compound, it is difficult to produce a pure aluminum nitride sintered product without utilizing sintering aids or a hot-press sintering method.
U.S. Pat. No 4,746,637 discloses that a combination of Y.sub.2 O.sub.3 and CaO as sintering aids. That is, a mixture of an AlN powder and the sintering aids is densely sintered at a temperature of 1650.degree. C. or above to obtain an AlN sintered product. However, thermal conductivity of the AlN sintered product is only 100 W/m.multidot.k or less. For using as a substrate in a power module, etc., an AlN sintered product having a high thermal conductivity of 120 W/m.multidot.k or more is desired.
Japanese Early Publication [KOKAI]No. 03-146471 discloses that a combination of Y.sub.2O.sub.3 and LaB.sub.6 is utilized as the sintering aids. In addition, Japanese Early Publication [KOKAI]No. 03-197366 discloses that a combination of CaO and LaB.sub.6 is utilized as the sintering aids. Though AlN sintered products obtained by using these combinations of the sintering aids show a high thermal conductivity, a high sintering temperature of 1900.degree. C. or above is required, so that it is necessary for expensive high temperature sintering furnace and fittings such as a setter capable of using at the high temperature. Additionally, there has been a problem of an expensive energy cost for producing the AlN sintered products.
In case of utilizing the hot-press sintering method for producing the AlN sintered product, though the sintering temperature can be lowered, a size or shape of the AlN sintered product would be limited.